This invention relates generally to nondestructive testing of pipe and tubing using magnetic flux leakage inspection, and more particularly the invention relates to a probe for use in such testing.
The use of magnetic flux leakage inspection of pipes and tubing is well known in the art of nondestructive testing. A magnetic field is established in the material undergoing inspection, and a sensor detects changes in flux patterns at the surface of the material due to cracks, pits, or wall thickness variations. Heretofore, eddy current coil detectors have been employed to detect sharp responses due to defects such as cracks and pits, while Hall effect sensors have been employed to detect more gradual responses due to wall thinning, for example.
In recent years, special ferritic alloys and carbon steel tubes have been employed in heat-exchanger tubing. These materials present a problem for in-service inspection due to their highly ferromagnetic properties. These properties severely limit the application of standard eddy current techniques which are widely used for non-magnetic tubing inspection. Presently, eddy current sensors with magnetic saturation are used on a limited basis for ferrous tube inspection. However, this method has several disadvantages including detecting false indications due to permeability variations, insensitivity to gradual types of tube defects, need for tube cooling during inspection, and complicated and expensive probes and instrumentation. Thus, there is a recognized, industry-wide need for a better method and apparatus for nondestructive ferrous tube inspection.